A Russian nuclear-powered attack submarine armed with long-range cruise missiles operated undetected in the Gulf of Mexico for several weeks and its travel in strategic U.S. waters was only confirmed after it left the region, the Washington Free Beacon has learned.

It is only the second time since 2009 that a Russian attack submarine has patrolled so close to U.S. shores.

The stealth underwater incursion in the Gulf took place at the same time Russian strategic bombers made incursions into restricted U.S. airspace near Alaska and California in June and July, and highlights a growing military assertiveness by Moscow.

The submarine patrol also exposed what U.S. officials said were deficiencies in U.S. anti-submarine warfare capabilities—forces that are facing cuts under the Obama administration’s plan to reduce defense spending by $487 billion over the next 10 years.

The Navy is in charge of detecting submarines, especially those that sail near U.S. nuclear missile submarines, and uses undersea sensors and satellites to locate and track them.

The fact that the Akula was not detected in the Gulf is cause for concern, U.S. officials said.